This invention relates to digital measuring devices which have multiple functions. In the testing of electrical components of a device under test, including circuit components, it is desirable to use a single instrument for performing measurements of a number of parameters such as voltage, current, resistance and capacitance. These meters are referred to as multimeters. With the advent of digital technology, digitally controlled multimeters have been developed and are known as digital multimeters (DMM). Known digital multimeters will, under the control of a microprocessor or other program controller, control the measurement of a specific characteristic of a device under test. When the measurement of the parameter has been completed, the result is either printed or, more commonly, displayed on a display such as a liquid crystal or other display screen. In certain prior art devices, a microprocessor included with the device only functioned to control the liquid crystal or other screen of the display device. Digital technology was not used to control the selection of parameters to be measured of the device under test. More recently, however, as shown in U.S. Pat. Nos. 4,864,512, issued Sep. 5, 1989 to K. A. Coulson et al., and entitled "Measurement Apparatus with Plural Displays of Measured Parameter and Selectable Function Thereof" and U.S. Pat. No. 4,794,383, issued Dec. 27, 1988 to Karel Havel, and entitled "Variable Color Digital Multimeter", the microprocessor or other program controller has been used to both drive the display as well as to control at least some of the functions in the apparatus in the course of measuring one or more predetermined parameters of a device under test.
A problem common both to digital multimeters as well as to older version analog multimeters is that of potential overloading of the sensitive circuits within the multimeter. Such overloading may come as a result of connection by the user of the apparatus to a voltage or current source with a magnitude greater than the capability of the circuitry within the multimeter to handle. For example, in one type of prior art multimeters, prior to measuring a parameter of a device under test, the user must attach electrical leads, usually two, to the device under test. The other ends of the leads are connected to the multimeter, which has a plurality of lead sockets to accept the leads from the device under test. The sockets will correspond to a range of a parameter, such as voltage. A typical voltage range for one socket may be from 0 to 15 volts, whereas the next socket may be for voltage ranging from 0 to 150 volts. If the user were to connect the leads to activate the circuit within the multimeter corresponding to measurement of the voltage within the 0 to 15 volt range, and if the user was further unaware that the voltage in the device under test exceeded 1000 volts, connection of the leads and activation of the multimeter would in all likelihood result in damage and burnout of the circuit components, making the multimeter useless and in need of replacement.
Other multimeters may employ a rotary switch in lieu of the lead sockets. The rotary switch is used, in lieu of movement of leads into sockets on the multimeter body, to select the range of voltages or current within which the user expects the device under test to fall. Once again, if the device under test exceeds the range selected by the user, damage and potential destruction of the rotary switch and the circuitry within the multimeter is a very real possibility.
Another shortcoming of prior art multimeters is their inability to readily measure more than one parameter of a device under test without either moving leads into different sockets in the multimeter body in the first type described above or by moving the rotary switch to the appropriate setting in the second type described above. Thus, in the prior art, if a user measured the voltage of a device under test and then wished to measure the current of that same device, the user would, while holding the multimeter in one hand, use the other hand to turn the rotary switch :from the voltage setting to the current setting. Furthermore, in the prior art devices, with the need for a rotary or slide switch to allow for the selection of ranges and parameters to be measured, the overall expense of the multimeter is increased. The use of mechanical rotary or slide switches does not fully utilize the capability of a microprocessor or other program controller (driving the display screen only) to perform the function of shifting parameters to be measured (from voltage measurement to current measurement, for example). If too high a voltage or current is passed into the multimeter due to the user incorrectly setting or positioning the rotary switch, the digital circuitry will be damaged, often requiring replacement of the entire multimeter.